1. Field of the Invention
The present invention relates to a recording medium reader/writer system for writing and reading data in and from a recording medium such as a non-contact IC card and a non-contact IC card system utilizing the same.
2. Discussion of Related Art
Though the conventional IC card is of a contact type in which the card has an electrical contact and the card is brought into connection to an IC card reader/writer system (hereinafter simply referred to as reader/writer) by bringing the electrical contact of the card into contact with an electrical contact of the reader/writer, a non-contact IC card has been proposed in which coils are used in lieu of the electrical contacts and the card is magnetically coupled to a reader/writer by the coils in a non-contact fashion, as disclosed by, for example, U.S. Pat. No. 5,113,184 issued May 12, 1992 (or its corresponding European Patent Application No. 88308709.0 filed Sep. 26, 1988).
The following explanation will be made of a non-contact IC card. Therefore, a non-contact IC card will merely be referred to as an IC card. In the U.S. Pat. No. 5,113,184, the IC card has a coil made of a metal loop and electronic switching means for making short-circuit/open-circuit between opposite ends of the coil and the reader/writer has a transmitter coil and a receiver coil which are coaxially provided at a predetermined interval. The reader/writer and the IC card are connected to each other by inserting the coil of the IC card between the transmitter and receiver coils of the reader/writer so that the coils are magnetically coupled.
In the case where data is to be read from the IC card, a sinusoidal or rectangular high frequency signal is supplied from an oscillator circuit to the transmitter coil and the electronic switching means of the IC card is turned on and off in accordance with "1" and "0" bits of data. Thereby, the amplitude of a high frequency signal induced in the receiver coil by magnetic fluxes generated from the transmitter coil by the high frequency signal from the oscillator circuit makes a change. The reader/writer reads data from the IC card by amplifying, rectifying and amplitude-detecting the high frequency signal having such a change.
IC cards need a supply voltage for driving a microcomputer incorporated therein, an external memory or the like. In a contact IC card, an electrical contact for the exclusive use for power supply is provided and a DC supply voltage is supplied from a reader/writer through this electrical contact. In a non-contact IC card, a high frequency signal is sent from a reader/writer through coils and a predetermined supply voltage is obtained by processing the high frequency signal by a rectifying/smoothing circuit and a regulator.
In the case where data is to be transmitted from a reader/writer to an IC card, a driver circuit drives a coil by a high frequency signal the amplitude, frequency or phase of which is modulated in accordance with the data. On the other hand, in the case where the reader/writer reads data from the IC card, the reader/writer reads the data in the above-mentioned manner, that is, in such a manner that the driver circuit drives a coil by a high frequency signal having a fixed amplitude and a change in amplitude of a high frequency current of the coil caused by the turn-on and turn-off of electronic switching means in the IC card is detected.
In the prior art reader/writer, however, since a general driver circuit using the existing linear amplifier or the like is used as the above-mentioned driver circuit, there is a problem that a power consumption of the driver circuit becomes too large. On the other hand, means for detecting the high frequency current of the coil includes current to voltage conversion means, for example, a resistor having a large resistance value, by which the detection is made from a voltage drop corresponding to the amplitude of the high frequency current. Therefore, in reading data from the IC card, the amplitude of a high frequency voltage supplied to the coil may vary in accordance with the turn-on and turn-off of the electronic switching means in the IC card.
On the IC card side, a predetermined supply voltage is necessary even when the reader/writer reads data from the IC card. This supply voltage is obtained from the high frequency voltage supplied from the reader/writer, as mentioned above. However, if the high frequency voltage does not have a large amplitude, a predetermined or stable supply voltage becomes unobtainable on the IC card side when the amplitude of the high frequency voltage varies in accordance with the turn-on and turn-off of the electronic switching means corresponding to data to be transmitted from the IC card to the reader/writer. Accordingly, it is required to obtain a predetermined supply voltage even at the minimum amplitude of the high frequency voltage. Namely, it is required that the amplitude of the high frequency voltage to drive the coil is made sufficiently large. This is attended with an excess power consumption.
On the other hand, if the amplitude of the high frequency voltage is small, there is a problem that the S/N ratio upon data detection becomes insufficient and sure data detection is therefore impossible. Accordingly, in the conventional system, it is required that the driver circuit in the reader/writer outputs a high frequency voltage having a large amplitude enough for compensation for a large voltage drop in the current detecting means. Thus, it is also required that a supply voltage for operating the driver circuit is sufficiently high. As a result, there is a problem that a power consumption of the reader/writer becomes large and a power supply circuit and hence the reader/writer becomes large in size.
In the conventional non-contact IC card, when an abnormal condition occurs, a CPU of the IC card detects the abnormal condition and sends the result of detection to a reader/writer. On the reader/writer side, a host computer judges the abnormal condition of the IC card (and further the kind of abnormality) from the result of detection sent from the IC card side.
In such an IC card, there may be the case where a transceiver circuit falls into an abnormal condition and the IC card is operated in a state in which the transceiver circuit is in the abnormal condition. In the worst case, there may be a fear that the IC card is destroyed.
On the other hand, the assignee of the present application has proposed, in Japanese Patent Application No. 3-205693 filed Jul. 23, 1991, a system in which means for detecting an abnormal condition is provided in an IC card and information detected by the detecting means is transmitted to a reader/writer in a manner similar to a data.